Nanocrystal Layer Deposition: Surface-Mediated Templating of

Department of Earth and Planetary Sciences, University of New Mexico. , #. Current address: National Renewable Energy Laboratories, Golden, CO 80401. ...
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J. Phys. Chem. C 2009, 113, 16329–16336

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Nanocrystal Layer Deposition: Surface-Mediated Templating of Cadmium Sulfide Nanocrystals on Zinc Oxide Architectures Erik D. Spoerke,*,† Matthew T. Lloyd,‡ Yun-ju Lee,† Timothy N. Lambert,§ Bonnie B. McKenzie,| Ying-Bing Jiang,⊥ Dana C. Olson,#,‡ Thomas L. Sounart,¶,† Julia W. P. Hsu,‡ and James A. Voigt† Department of Electronic and Nanostructured Materials, Surface and Interface Sciences, Fuels and Energy Transitions, and Materials Characterization, Sandia National Laboratories, Albuquerque, New Mexico 87109, and Department of Earth and Planetary Sciences, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: June 2, 2009

The integration of zinc oxide (ZnO) and cadmium sulfide (CdS) has shown promise in applications such as sensing and photovoltaics. We describe here a room temperature, aqueous synthetic strategy to selectively grow nanocrystalline CdS on ZnO. In particular, we describe an experimentally simple process that selectively grows a conformal nanocrystalline coating only 1 nanocrystal (10-20 nm) thick on planar and three-dimensional extended ZnO structures. We explore the synthesis, characterization, and mechanisms involved in the formation of this composite material, and we demonstrate a promising optical response from these composite structures that may prove valuable for future optoelectronic applications. Introduction The integration of various semiconductors such as zinc oxide (ZnO) and cadmium sulfide (CdS) continues to show promise in developing technologies ranging from sensing to photovoltaics. Composite nanorods composed of CdS shells covering ZnO cores have shown improved sensing capabilities,1,2 and Ayyub et al.3 previously demonstrated enhanced photoluminescence from composite quantum dot structures. In photovoltaics, interfacing a ZnO window with an n-type CdS layer has improved the performance of high-efficiency thin-film solar cells including Cu(In,Ga)Se2 (CIGS) cells,4-7 and InP/CdS/ZnO cells.8 In all of these applications, the proper functioning of these opto-electronic materials requires the controllable formation of intimately interfaced ZnO and CdS. In the present work, we describe a room temperature, solution phase process that grows a single, uniform layer of CdS nanocrystals on ZnO nanostructures, integrating these two materials into a chemically and electronically intimate composite. A number of other methods have been described for the growth of CdS on ZnO. In some cases, such as the sensing examples described above, CdS/ZnO composites are synthesized in particle form through solution phase syntheses and the materials must subsequently be applied to a surface or device structure. This approach introduces a variety of complications related to controlling nanoparticle assembly, and ensuring intimate contact with the device platform. Naturally, the ability * To whom correspondence should be addressed. Phone: 505-284-1932. Fax: 505-844-9781. E-mail: [email protected]. † Department of Electronic and Nanostructured Materials, Sandia National Laboratories. ‡ Surface and Interface Sciences, Sandia National Laboratories. § Fuels and Energy Transitions, Sandia National Laboratories. | Materials Characterization, Sandia National Laboratories. ⊥ Department of Earth and Planetary Sciences, University of New Mexico. # Current address: National Renewable Energy Laboratories, Golden, CO 80401. ¶ Current address: Intel Corporation, Chandler, AZ 85248.

to grow these materials in situ on a device structure or sensing platform would be of great value, and processes such as sublimation/condensation, electrodeposition, screen printing, spray pyrolysis, and chemical bath deposition5,9-11 have been developed to allow CdS growth on substrates. Each of these methods has inherent limitations, however. Directional growth methods such as sputtering, screen printing, or spray pyrolysis can present problems uniformly coating complex, extended (three-dimensional) film structures. Electrochemical methods are subject to the limitations of substrate conductivity and stability, and sublimation/condensation requires specialized equipment and elevated temperatures to achieve uniform CdS growth. Chemical bath deposition (CBD) is a solution phase growth process that has proven to be an attractive alternative to these other approaches, providing uniform CdS films. Most often, CdS films are grown in a heated (usually 90 °C or less) alkaline solution containing a cadmium salt, an organic sulfur source, typically thiourea or thioacetamide, an added base, and a chelating agent to control metal ion hydrolysis.12-17 Although this is a widely utilized process, it can produce homogeneous, nonspecific growth, it involves a series of complex reactions that remain imperfectly understood, and the alkaline nature of the reaction makes it less compatible with base-soluble substrates, such as ZnO. It is generally accepted that the alkaline environment of these reactions is important to the decomposition of the thiourea or thioacetamide sulfur sources.12,18,19 As a result, alternative CBD reactions performed in more acidic media have required either using a substitute sulfur source,20 or external stimuli such as sonication, irradiation with ultraviolet light, or electrochemical activation to promote CdS growth.2,21,22 Here, we describe a simplified process we call NanoCrystal Layer Deposition (NCLD) that utilizes room temperature, slightly acidic aqueous chemistry to selectively grow CdS on nanostructured ZnO. This unusual growth process requires no external bases or chelating agents to control growth; CdS growth is controlled by the surface chemistry of the ZnO substrate, which regulates both the initiation and termination of CdS

10.1021/jp900564r CCC: $40.75  2009 American Chemical Society Published on Web 08/21/2009

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growth. This process results in the selective formation of a single layer of CdS nanocrystals, grown conformally around a variety of ZnO architectures. In addition to describing the synthesis and characterization of the these composite structures, we explore the mechanisms of the growth process, examining the interactions between the ZnO surface, the cadmium salts, and the organic sulfur source, thioacetamide. Finally, we demonstrate photocurrent generation and present a photocurrent action spectrum for this system as a measure of potential utility of these heterostructured CdS/ZnO composite materials in optoelectronic applications. Methods and Materials Growth of Zinc Oxide (ZnO). ZnO thin films were prepared by several previously reported solution-based methods, and the syntheses and characterizations of these materials will be described at a minimum in order to focus the present work on the CdS/ZnO composites. Preparations were identical on all substrate types. Sol-gel ZnO. ZnO sol-gel films were spin-coated from a solution of 0.75 M zinc acetate, 0.75 M ethanolamine in 2-methoxyethanol. Films were spun at 2000 rpm for 60 s and annealed at 300 °C for 10 min in air. Multiple coats were sequentially applied by repeating this process. ZnO Nanorod Arrays (NRAs). ZnO nanorod arrays were grown from seed layers of either dispersed nanoparticles of ZnO23 or the sol-gel film described above. Particle-seeded substrates were incubated overnight at 60 °C in aqueous 0.25 M Zn(NO3)2 and 0.25 M hexamethylenetetramine (HMT). Sol-gel seeded substrates were incubated in 0.25 M Zn(NO3)2 and 0.25 M HMT for 1 h at 92.5 °C. Samples were removed from the reaction solution, rinsed thoroughly with deionized water, and air-dried before CdS growth. Complex ZnO Architectures.24 “Primary” ZnO rods were grown by incubating clean, unseeded glass slides in aqueous 25 mM Zn(NO3)2 and 25 mM HMT overnight at 60 °C. These substrates were rinsed and air-dried before introducing them to a second aqueous growth solution containing 0.25 M Zn(NO3)2, 0.25 M HMT, and 0.43 mM diaminopropane (DAP) for 6 h at 60 °C. Samples were rinsed with deionized water and dried before continuing with CdS growth. Growth of patterned ZnO nanorods was performed according to methods described by Hsu et al.25 Briefly, polydimethylsiloxane (PDMS) stamps were used to apply a “negative” selfassembled monolayer (SAM) of acid-thiols to a silver surface. When incubated in aqueous 25 mM Zn(NO3)2 and 25 mM HMT for 2-3 h at 60 °C, densely packed, vertical zinc oxide rods grew in the patterned spaces of silver not covered by the acidic thiol self-assembled monolayer. Growth of Cadmium Sulfide (CdS) on ZnO. CdS was grown by incubating the ZnO-coated surfaces in an aqueous solution of 10 mM Cd(NO3)2 and 10 mM thioacetamide at room temperature for 5-60 min. CdS growth was indicated by the emergence of a yellow color on the sample surface. Samples were then rinsed thoroughly with deionized water and air-dried. Testing of growth on alternative materials involved either immersing a substrate, such as silicon, into the CdS growth solution or adding powders of the material to the growth solution. Substrates were removed and rinsed with water while powdered samples were washed 3× with deionized water, centrifuging to collect the washed samples between each rinse. Powders were ultimately dried by vacuum desiccation prior to examination by X-ray diffraction.

Spoerke et al. Characterization. Samples on transparent substrates were characterized by transmission UV-vis spectroscopy, using an Ocean Optics USB2000 spectrometer and an Ocean Optics ISSUV-vis light source. Characterization by X-ray diffraction (XRD) was performed on a thin film diffractometer (Rigaku), using copper KR radiation at 40 kV and 30 mA. Scanning electron microscopy (SEM) was performed on dried samples, sputtered with a thin layer of gold-palladium before analysis in a Zeiss scanning electron microscope at 5 kV. To examine these materials by transmission electron microscopy (TEM), a clean razor blade was used to scrape CdS/ZnO nanostructures from the surface on which the materials were grown and deposit them on a holey carbon-coated copper TEM grid (SPI supplies) before examination by TEM. Solution NMR data were obtained on a 400 MHz Bruker NMR spectrometer at ambient temperature. Samples were prepared by simply taking a small aliquot from the reaction media, where water was replaced with deuterium oxide as the reaction solvent. All spectra were referenced to residual water. External quantum efficiency measurements were performed on bilayer CdS/ZnO composite films with use of a HORIBA Jobin Yvon Triax 180 monochromator. The monochromatic illumination intensity was monitored with a NREL calibrated silicon photodiode, while the photocurrent from the device was recorded with a Stanford Research SR810 lock-in amplifier operating at a chopping frequency of 40 Hz. Two layers of sol-gel ZnO were deposited on patterned indium tin oxide (ITO) substrates and CdS was grown on these ZnO-coated substrates as above. To avoid shorting, an aqueous suspension of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (Sigma) was spun on top of the CdS film twice at 4000 rpm and annealed at 120 °C for 10 min. PEDOT:PSS was carefully wiped away from device contacts immediately after spinning. Thermal evaporation was then used to apply a 100 nm thick silver top electrode. Results and Discussion When CdS was grown on ZnO-coated substrates, samples visually changed color from the natural white or transparent color of ZnO to the bright yellow color expected for CdS. Closer examination of the substrates by scanning electron microscopy (Figure 1) revealed that the yellow color was due to the formation of a 10-20 nm thick nanoparticle coating of CdS on the ZnO nanostructures. The series of micrographs in Figure 1 illustrate a wide range of ZnO architectures onto which CdS was grown. In the top row, an array of vertical ZnO nanorods are shown as grown (Figure 1a) while Figure 1b shows similar rods after CdS growth. The ZnO rods in panel b retained their hexagonal geometry, but the roughened texture of the nanoscale CdS coating on these rods is also clearly visible. Figure 1c shows a higher magnification image of this nanoparticle coating. Nanocrystals of CdS approximately 10-20 nm in diameter are densely packed in a conformal layer over the hexagonal template of ZnO. Panels d-f in Figure 1 show a number of other conformally CdS-coated ZnO nanostructures including a sol gel film (d), patterned ZnO rods grown on a silver/silicon substrate (e), and complex branched ZnO architectures (f). Figure 2 shows the energy dispersive X-ray spectrum (EDX) and the powder X-ray diffraction (XRD) pattern of CdS grown on an array of ZnO rods on a glass slide and single crystalline silicon wafer, respectively. The presence of Cd, S, Zn, O, and Si in the EDS spectrum of Figure 2a is consistent with the formation of CdS and indicates that the ZnO was not consumed

Surface-Mediated Templating of CdS Nanocrystals on ZnO

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Figure 1. Scanning electron micrographs of CdS-coated nanostructures. (a) An uncoated ZnO nanorod array, grown from a particle seed layer. The inset shows a high magnification image of an uncoated ZnO nanorod. Slight texturing of the surface from Au-Pd coating is visible. (b) A ZnO nanorod array like that in panel a, after growth of conformal film of nanocrystalline CdS. (c) A high magnification image of a hexagonal ZnO nanorod coated with a CdS nanocrystal film. CdS nanocrystals are approximately 10-20 nm across. (d) A CdS nanoparticle film grown on a ZnO sol-gel film. (e) Patterned circles of CdS-coated ZnO nanorods on a silver surface. The inset shows a higher magnification image of an isolated rod from one of these patterned structures. (f) A complex, branched ZnO structure, uniformly coated with a CdS nanoparticles film. A higher magnification inset shows textured CdS coating on the branched ZnO structure.

Figure 2. (a) An energy dispersive X-ray spectrum of Figure 1b, showing the presence of both ZnO and CdS, grown on a glass slide. (b) A powder X-ray diffraction pattern of CdS grown on an oriented ZnO nanorod array extending from a silicon wafer. Diffraction planes are indicated for ZnO and CdS. Intense (002) peaks indicate preferential c-axis orientation of both ZnO and CdS.

or dissolved during the CdS growth. The XRD spectrum reveals diffraction peaks from the silicon substrate, the ZnO nanorods, and the nanocrystalline CdS formed on ZnO surfaces. Estimates of the CdS particle size calculated by applying the Scherrer equation26 to the (002) peak of CdS suggest that the CdS particles are approximately 11 nm in diameter, a value consistent with observations by electron microscopy. The strong (002) reflection for CdS around 27° in the XRD pattern also indicates a preferential c-axis orientation. Significant lattice mismatch along the c-axes of CdS and ZnO (CdS002 ) 3.36 Å, ZnO002 ) 2.60 Å) (PDF no. 36-1451 for ZnO, PDF no. 41-1049 for CdS) makes it unlikely that the c-axis oriented ZnO is epitaxially templating the CdS. Studies of the CdS/ZnO interface by highresolution transmission electron microscopy were unable to provide conclusive evidence to support or dismiss this idea. Previous reports have also indicated strong c-axis orientation of CdS grown by CBD,16,27 and further investigation of this system may provide greater insight into the nature of the crystallographic orientation observed here. In Figure 3, a series of TEM micrographs shows a bare ZnO nanorod (a) and both brightfield (b) and darkfield (c) images of a CdS-coated ZnO nanorod physically removed (scraped with a razor) from a substrate like that shown in Figure 1b. In Figure 3b, one can clearly see the nanocrystals

of CdS, again 10-20 nm across, grown densely around the ZnO nanorod. The darkfield image in Figure 3c shows the brightly speckled polycrystalline CdS coating, 10-20 nm thick, contrasted against the single-crystal ZnO nanorod core. The EDS spectrum of the ZnO/CdS nanocrystal structure (Figure 3b,c) confirms that the texture seen in these images is due to introduction of CdS, not an artifact, such as etching of the ZnO nanorod. The scanning transmission electron micrograph (STEM) shown in Figure 3d shows the end of a CdS-coated ZnO rod where the end of the ZnO rod has been deliberately etched away with 10 mM hydrochloric acid, and part of the resulting CdS “shell” has broken away. Individual nanocrystals of CdS, 10-20 nm in diameter, are visible on the “back” of the CdS shell, while the opposing black and white arrows indicate the thickness of the CdS shell is only approximately 10-20 nm thick, corresponding to a single nanocrystal shell thickness. Finally, Figure 3f shows a crosssectional TEM image of a CdS-coated sol-gel ZnO film. The dense, nanocrystalline CdS film is visible on top of the relatively less-dense ZnO coating. Again, the CdS coating is shown to be 20 nm or less thick, confirming the thickness of the CdS film is the same on both nanorods and the sol-gel film. It should be noted when comparing the nanorods in Figure 3a-d, that there is considerable variation in the

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Figure 3. Transmission electron micrographs of (a) a single-crystal ZnO nanorod (brightfield), (b) a ZnO nanorod densely coated with CdS nanocrystals (brightfield), and (c) the CdS-coated ZnO nanorod from panel b in darkfield. The image in panel c clearly shows the bright nanocrystals of CdS surrounding the dark ZnO nanorod core. Images in panels b and c show that nanocrystals, approximately 10-20 nm in size, form a coating only 10-20 nm thick on the ZnO nanocrystal. (d) A scanning transmission electron micrograph of a broken CdS “shell” formed by acidic dissolution of the ZnO nanorod core. The arrows designate the thickness of the shell at 10-20 nm. (e) An energy dispersive X-ray spectrum of the composite nanorod in panels b and c, confirming that the textured surface formed on the ZnO nanorod contains CdS. (f) A brightfield cross-sectional TEM image of a CdS coating on sol-gel ZnO coating. The sol gel ZnO was deposited on ITO-coated glass.

diameters of the nanorods shown, where the ZnO nanorod diameters may vary from less than 100 nm to nearly 200 or 250 nm, a consequence of nanorod density variation on the glass substrate. Figure 4 shows high-resolution images of CdS nanocrystals grown on single-crystal ZnO nanorods. The image in Figure 4a clearly shows the lattice fringes corresponding to the (002) plane of the c-axis oriented ZnO nanocrystals. The white arrow in Figure 4a indicates the orientation of the ZnO c-axis. Figure 4b shows a collection of CdS nanocrystals grown along the ZnO and lattice fringes for hexagonal CdS nanocrystals are visible in this micrograph. The magnified image in Figure 4c reveals the hexagonal character and confirms the lattice spacings (4.14 Å) expected along the [100] and [010] directions of CdS with a wurtzite crystal structure. The magnified image in Figure 4d shows lattice fringes spaced as expected for the (002) planes of the CdS (3.36 Å). The images in Figures 3 and 4 clearly show that CdS grew to form a nanocrystalline coating over the surfaces of ZnO. Further investigation of the system, however, showed that the CdS was selectiVe for the ZnO. Over the course of several hours at room temperature, CdS growth was not observed on glass, silicon, silver, gold, indium tin oxide (ITO), aluminum oxide, tin oxide, iron oxide (Fe2O3), copper oxide (CuO), or lead oxide. Figure 5 illustrates the selectivity of this process. Figure 5a shows a glass slide, coated with white “stripes” of ZnO nanorods, immersed in the clear reaction solution containing cadmium nitrate and thioacetamide. As the CdS growth progresses, the white ZnO stripes turn bright yellow, indicating the growth of CdS on the ZnO stripes. There is no visible growth of the CdS either on the glass slide itself or in the growth solution surrounding the sample. This macroscopic observation is also supported on the microscale in the scanning electron

micrograph of Figure 5c. EDX analysis was used to locate CdS growth on ZnO nanorods, patterned on a silver-coated silicon surface.25 These spectra, shown in Figure 5d, clearly show that the CdS grew on the ZnO, but not on the surface surrounding the nanorod. The selectivity of CdS growth apparent in Figure 5 also suggests that the ZnO is necessary for CdS growth under these conditions. To test this idea, the CdS growth reaction was interrogated by proton nuclear magnetic resonance spectroscopy (1H NMR), replacing water with deuterium oxide as the reaction solvent. When cadmium nitrate and thioacetamide were combined in the absence of ZnO, no CdS formed, but the 1H NMR data showed that a small quantity of acetamide (δ ) 1.92 ppm) was formed during what was likely a Cd(OH)2-mediated decomposition of thioacetamide (δ ) 2.37 ppm).12,18,19 Although both acetamide28-30 and acetonitrile21 byproducts of thioacetamide have been reported in the literature, FTIR analysis of our reaction solutions showed acetamide to be the byproduct produced in the present case. IR spectra of the CdS-ZnO reaction solution showed the absence of a strong absorbance band around 2253 cm-1, expected for acetonitrile, but did show the emergence of an absorbance band around 1401 cm-1 that differentiates acetamide from the thioacetamide starting material. In the absence of a cadmium salt, the decomposition of thioacetamide was not observed; no acetamide byproduct was observed by 1H NMR, even when the sample was heated to 60 °C in either the absence or presence of ZnO at pH 5.5. These observations are consistent with a widely acknowledged understanding that Cd(OH)2 initiates the decomposition of chalcogenide precursors such as thiourea and thioacetamide,12,18,19 though these reactions are typically performed in alkaline environments where Cd(OH)2 is a prevalent species.31 The

Surface-Mediated Templating of CdS Nanocrystals on ZnO

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Figure 4. High-resolution transmission electron micrographs of CdS nanoparticles on single-crystal ZnO nanorods. The white arrow in panel a indicates the c-axis direction of the ZnO and lattice fringes from the (002) planes of the ZnO single crystal are visible along the length of the ZnO crystal. The image in panel b shows a collection of CdS nanocrystals on the ZnO substrate. Magnified images in panels c and d show the lattice fringes corresponding to the hexagonal crystal structure of the CdS.

Figure 5. (a) Photograph of glass vials containing CdS reaction solution and ZnO nanorods grown on macroscopic patterns of ZnO seeds. (b) The vials from panel a after 30 min of incubation in CdS growth solution at room temperature. The yellow color from CdS growth clearly shows macroscopic selectivity of CdS growth on ZnO. No CdS is observed on the glass nor in the reaction solution itself. (c) A scanning electron micrograph of a patterned ZnO nanorod grown on a silver/silicon substrate and coated with CdS. (d) Energy-dispersive X-ray spectra representative of points labeled in panel c indicating that the CdS growth is selective for ZnO on the submicrometer scale.

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Figure 6. (a) UV-vis spectra of sol-gel ZnO and sol-gel ZnO after CdS growth. (b) Growth rate, as determined by absorbance at 450 nm, of CdS grown on sol-gel ZnO (0), 135 nm long ZnO nanorods (O), and 350 nm long ZnO nanorods (2). Growth on all substrates follows the same trend of rapid CdS growth, followed by cessation of growth. CdS growth on ZnO sol-gel plateaus after 5-10 min; however, CdS growth on ZnO rods does not plateau until 25-30 min.

overall reaction that would be predicted for CdS growth under these conditions is given by eq 1: CH3C(S)NH2 + Cd(OH)2 f CH3C(O)NH2 + CdS + H2O

(1) In the present work, however, the CdS growth process occurs at pH ∼5.5, a pH at which a hydrolysis diagram for cadmium indicates that Cd(OH)2 may be present, but as a minor species.31 This speciation would explain the NMR data indicating that some thioacetamide was decomposed in the presence of Cd(NO3)2 alone. Because the amount of Cd(OH)2 present was small at this pH, however, insufficient thioacetamide would have degraded to drive homogeneous CdS precipitation. In contrast, when ZnO was introduced to the thioacetamide/ cadmium nitrate solution, yellow CdS precipitated selectively on the ZnO substrate within minutes. Integration of the acetamide peaks from Cd2+-containing reaction solutions that contained a ZnO-coated substrate showed that the acetamide peak, formed during thioacetamide degradation, increased by a factor of 3 over a Cd2+-containing reaction solution that contained no ZnO substrate. Since our NMR experiments showed that the ZnO substrate alone was unable to degrade the thioacetamide, we conclude that the ZnO must be forming an activated cadmium hydroxide complex on its surface capable of decomposing the thioacetamide. At pH 5.5, well below the isoelectric point of ZnO (∼9)31,32 one would expect a high density of bridging hydroxyls on the ZnO surface. Again, our NMR experiments, in agreement with previous reports,12 have shown that this hydroxylated ZnO surface alone was unable to induce the decomposition of thioacetamide. The role of the hydroxylated ZnO surface in the present case, then, is to capture the partially hydroxylated cadmium moieties on the ZnO surface. Bridging hydroxyls on the ZnO surface are expected to complex Cd2+, CdOH+, and other partially hydroxylated cadmium complexes in solution. The resulting surface of ZnO, heavily decorated with surface-bound cadmium hydroxide complexes, is capable of reacting with thioacetamide to form CdS (eq 1), nucleated directly on the ZnO. Further details about the nature of the CdS growth on ZnO were revealed by UV-vis spectroscopy. Figure 6a shows the absorbance spectra for a glass slide coated with a sol-gel film before and after 30 min of CdS growth. The ZnO curve shows strong absorbance centered around 375 nm. While this ZnO absorbance is still evident in the CdS/ZnO composite, strong additional absorbance from the CdS is also visible, beginning around 510 nm. By comparison, no absorbance from CdS was observed on a clean, glass slide after more than 60 min of growth

time, confirming that the CdS grows neither heterogeneously on glass nor homogeneously in solution. Using the CdS absorbance to monitor the amount of CdS growing on ZnO substrates, we were able to observe the kinetics of the CdS growth. Figure 6b shows the change in the absorbance intensity from CdS growing on several ZnO substrates over the course of 60-90 min. Looking first at the planar ZnO films, we can clearly see a rapid rise in absorbance, followed by a marked leveling of intensity, suggesting that the CdS grows rapidly on the planar ZnO film for the first 5-10 min at which point growth largely ceases. Interestingly, the rate of CdS growth was invariant with respect to sample size. When the CdS-coated ZnO substrate was replaced with a fresh ZnO substrate, CdS growth resumed, indicating that the previous cessation of growth was not due to depletion of solution reagents. The cessation of CdS growth is determined by exposure to the cadmium hydroxide-decorated ZnO surface. In the early stages of the reaction, there is maximum ZnO surface exposed to promote CdS growth. As the ZnO surface becomes covered with growing CdS, however, the reaction slows, eventually ceasing when the reactive surface-bound cadmium hydroxide moieties are consumed or covered by the nanocrystal layer of CdS. The extended plateau of the CdS absorbance in Figure 6b suggests that the CdS nanocrystals are not sufficiently reactive with the thioacetamide to promote the continued heterogeneous crystal growth. This behavior produces the relatively uniform, dense layer of CdS nanocrystals on the ZnO, only a single nanocrystal thick. This surface-limited, selfterminating growth process, which we call Nanocrystal Layer Deposition (NCLD), is in many ways analogous to self-limiting, molecular growth processes such as atomic layer deposition (ALD). Just as with ALD, the selection of reaction precursors is important to this CdS NCLD process. The room temperature reaction was dependent on the use of thioactetamide as the source of sulfide ions. Control experiments in which thioacetamide was replaced with equimolar quantities of sodium sulfide did not produce selective, templated CdS growth; rather, bulk cadmium sulfide precipitated in the solution immediately upon introduction of the free sulfide ions to the cadmium-containing solution. This extremely rapid reaction prevented any cadmium sulfide from nucleating on the zinc oxide. In addition, other organic thiols, such as thiourea, were not sufficiently reactive at room temperature, failing to produce any CdS growth. Clearly, the kinetics of thioacetamide’s room temperature reactivity with cadmium hydroxides, both in solution and on the ZnO surface, are important to driving the selective nucleation and growth of the nanocrsytalline CdS.

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TABLE 1: Correlation between Variations in Surface Area of Different ZnO Nanostructures and the Absorbance of CdS Grown on the ZnO Surfaces ZnO surface

Φ

nanorod radius (nm)

nanorod height (nm)

surface area multiplier

absorbance of CdS (490 nm)

CdS absorbance ratio

planar sol-gel 135 nm rods 350 nm rods

1 0.25 0.25

N/A 10-12 10-12

0 135 350

1 6.9 16.2

0.031 0.21 0.51

1 6.8 16.4

NCLD proved particularly important for growth on threedimensional structures, such as the nanorods and branched structures seen in Figure 1. With the greatest exposure to the reaction solution, the tops of the ZnO rods, for example, would promote growth first. In an uncontrolled reaction, this early growth could have led to the formation of a CdS “cap,” preventing diffusion, and therefore growth, further down the rods. The gradual and self-limiting nature of the NCLD process, however, allowed CdS to grow uniformly over complex and extended ZnO structures. The importance of diffusion in this system is evident in the data shown in Figure 6b. The absorbance data show that CdS growth on ZnO rods (∼135 and ∼350 nm long) follows a trend similar to that on the planar structures, but the plateau in absorbance associated with growth cessation does not occur until approximately 25-30 min. This delay is believed to be due to the increase in time required for reactants to diffuse into and react with extended ZnO nanostructured films. Figure 6b also illustrates several other important points. The amount of CdS grown (as determined by absorption intensity) increases with ZnO rod length, and this increase correlates extremely well with the relative increase in surface area of the elongated rods, as illustrated in Table 1. The surface area multiplier (SAx) is calculated according to eq 2:

SAx ) 1 +

2HΦ r

(2)

where H is the height of the nanorods, measured by crosssectional SEM and absorbance intensity, Φ is the stereologically determined fraction of the substrate surface occupied by the tops of the hexagonal ZnO nanorods seen in SEM micrographs, and r is the radius of the nanorods, determined from plan-view SEM. The SAx, then, is the number by which the surface area of a planar substrate would be multiplied to determine the total surface area of the nanorod-covered surface. By comparison, the CdS absorbance ratio indicates the amount of CdS grown, relative to the amount grown on a planar ZnO substrate. The excellent correlation between the SAx and the CdS absorbance ratio suggests that virtually all of the nanorod surfaces are being covered by the CdS coating and furthermore that the uniform nanocrystal thickness (Figure 3) is preserved, regardless of

nanorod length. These data further support our proposed surfacedependent NCLD growth process. The selective and conformation nature of the CdS growth make this NCLD process particularly attractive for applications which require selective, in situ CdS growth. To test the functional viability of the CdS/ZnO composite for applications such as photovoltaics, we probed the optical response of a bilayer composite film. CdS was grown on ZnO-coated prepatterned ITO substrates. The top electrode consisted of a spincast layer of PEDOT:PSS followed by a 100 nm thermally evaporated Ag electrode. In this Schottky cell configuration, photogenerated holes are transported to the silver electrode (through the PEDOT contact), while electrons are transported from the n-type CdS through the n-type ZnO33,34 into the ITO electrode. External quantum efficiency (EQE) measures the efficiency with which incident photons are converted to electrons extracted to the external circuit. The EQE plot shown in Figure 7 shows that the nanocrystalline CdS coating, only 10-20 nm thick, generated photocurrent from light with wavelengths between 350 and 500 nm. The external quantum efficiency for this device peaked just over 13% at approximately 410 nm. Internal quantum efficiency (IQE) indicates the photon-toelectron conversion efficiency based on the fraction of incident light absorbed by the photoactive layers. This metric represents an arguably more realistic measure of the photocurrent generating capability of the absorbing layers for extremely thin planar bilayer devices utilized here. By normalizing the peak EQE signal according to the amount of light actually absorbed at 410 nm, we can estimate the IQE. On the basis of the measured absorbance of 0.11 at 410 nm, we determined that approximately 40% of the light at this wavelength was absorbed by the CdS layer in the EQE experiments.35 Accordingly, the IQE of photoelectron generation is estimated at 32-35%. These data suggest that the nanoscale CdS coating could be a valuable UVabsorbing component of a photovoltaic system and, perhaps more importantly, carrier transport across the intimate CdS/ZnO interface was good. Future studies will show how this CdS layer could be effectively incorporated into other photovoltaic systems containing elements with stronger visible light absorption. The simplicity, selectivity, and self-limiting nature of the growth approach and the stability of this inorganic configuration could make this an attractive materials option for future photovoltaic device development. Conclusions

Figure 7. External quantum efficiency for the ITO/ZnO/CdS/PEDOT/ Ag device.

We have described here a novel room temperature, aqueous process to selectively grow nanocystalline CdS onto ZnO in slightly acidic media without the need for cationic chelating agents or other base reagents, such as ammonia. This process relies on the formation of activated cadmium hydroxide complexes on ZnO surfaces. These surface-bound cadmium hydroxide complexes react with thioacetamide to selectively nucleate and grow nanocrystalline CdS on the ZnO surface. The cooperative interactions of the reagents produce CdS growth that is both selective for ZnO and self-limiting. The resulting

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CdS coatings, only 10-20 nm thick, form conformal nanoparticle films on ZnO nanostructures ranging from sol-gel films to extended films of three-dimensional nanostructures. Intimately interfaced with the ZnO on which it was grown, this thin CdS coating demonstrates a promising optical response, efficiently transferring photogenerated electrons into the ZnO in a photovoltaic device configuration. The simplicity, selectivity, and functionality of these CdS/ZnO composite structures make this a promising system for future optoelectronic applications. Acknowledgment. The authors acknowledge Dr. Bruce C. Bunker and Dr. David R. Wheeler for insightful technical discussions. This work was supported by Sandia’s Laboratory Directed Research and Development program and by the Division of Material Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. References and Notes (1) Du, N.; Zhang, H.; Chen, B.; Wu, J.; Yang, D. Nanotechnology 2007, 18, 115619. (2) Gao, T.; Li, Q.; Wang, T. Chem. Mater. 2005, 17, 887. (3) Ayyub, P.; Vasa, P.; Taneja, P.; Banerjee, R.; Singh, B. J. Appl. Phys. 2005, 97, 104310. (4) Caicedo, L.; Moreno, L.; Sandino, J.; Gordillo, G. Surf. ReV. Lett. 2002, 9, 1693. (5) Rau, U.; Schmidt, M. Thin Solid Films 2001, 387, 141. (6) Ruckh, M.; Schmid, S.; Schock, H. J. Appl. Phys. 1994, 76, 5945. (7) Saueberlich, F.; Fritsche, J.; Hunger, R.; Klein, A. Thin Solid Films 2003, 431-432, 378. (8) Saito, S.; Hashimoto, Y.; Ito, K. IEEE 1994, 1867. (9) Bonnet, D. CdTe Thin-Film PV Modules. In Practical Handbook of PhotoVoltaics: Fundamentals and Applications; Markvart, T., Castaner, L., Eds.; Elsevier: Oxford, UK, 2003; p 333.

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